Various color liquid crystal displays are conventionally used as color display devices having characteristics such as a flat shape and lightweight. As a liquid-crystal technology has been developed in recent years, a color liquid crystal device having a high contrast and a wide viewing angle characteristic has been developed and widely put to practical use as a mainstream of a large-size display.
Such color liquid crystal display devices that are widely used these days adopt, for example, (i) a twisted nematic mode (hereinafter referred to as a “TN mode”) in which an optical rotation of a liquid crystal layer is controlled by an electrical field so that a display is carried out, (ii) a birefringence mode (hereinafter referred to as an “ECB mode”) in which a birefringence of a liquid crystal layer is controlled by an electrical field so that a display is carried out, or the like mode.
However, there is a problem in which the color liquid crystal display device that uses these modes is not suitable for displaying moving image because an image lag phenomenon occurs or an outline of image is blurred due to a slow response speed.
In order to solve such a problem, many experiments for fastening a response speed of the color liquid crystal display have been conventionally carried out. Currently, a ferroelectric liquid crystal mode, an antiferroelectric liquid crystal mode, an OCB mode, or the like has been used as a liquid crystal mode having a fast response speed that is suitable for moving image display.
There has been known that, in these modes, the ferroelectric liquid crystal mode and the antiferroelectric liquid crystal mode have a lot of problems for practical uses because they have a low impact resistance due to a layered structure.
On the other hand, the OCB mode has been focused as a liquid crystal mode suitable for moving image display because the OCB mode that uses a usual nematic crystal (i) has a strong impact resistance, (ii) is available in a wide temperature range, and (iii) has a wide viewing angel and a fast response characteristic.
FIG. 14 schematically illustrates the OCB mode. In a liquid crystal display device to which the OCB mode is applied, a pair of transparent glass substrates 10 and 11 sandwich a liquid crystal layer 12 therebetween, and transparent electrodes 13 and 14 are respectively provided on the glass substrates 10 and 11 on a side of the liquid crystal layer 12, and alignment films 15 and 16 are respectively provided thereon. An alignment process is performed for the liquid crystal layer 12 by rubbing.
In a case where a color display is carried out in the liquid crystal display device, a color filter is provided on one of the glass substrates. In order that a liquid crystal is driven by active matrix, a gate bus line and a source bus line are provided on the other one of the glass substrates, and a TFT is provided at an intersection thereof. After the substrates are respectively provided, the substrates are bonded via a spherical spacer or a column spacer so that an arbitral gap is provided therebetween. A liquid crystal is injected in vacuum between the bonded substrates, or injected by a dropping method between the substrates when the glass substrates are bonded. In order that a viewing angle characteristic of display is improved, a wave plate (not illustrated) is bonded to either side or both sides of a liquid crystal cell, and a polarization plate (not illustrated) is externally bonded thereto.
The liquid crystal layer 12 that is right after the liquid crystal is injected is often aligned as illustrated in FIG. 15, which is called an initial alignment (splay alignment). When an intended voltage is applied to the electrodes 13 and 14 that are respectively provided above and below the liquid crystal layer 12, an alignment transition occurs in the liquid crystal layer 12 and the alignment is gradually changed to an alignment (bend alignment) illustrated in FIG. 14. When the liquid crystal layer 12 becomes the bend alignment as illustrated in FIG. 14, an alignment change of the liquid crystal makes a response rapidly. This allows the fastest display in modes that use nematic crystal. Furthermore, when a wave plate is provided in the liquid crystal display device, it is possible to realize a display state having a wide viewing angle characteristic.
As such, in the OCB mode, an alignment of a liquid crystal layer is a splay alignment while no voltage is applied, and a display is carried out in a state where the alignment of the liquid crystal layer is in a bend alignment. Consequently, in a liquid crystal display device to which the OCB mode is applied, when a display is carried out, a voltage is continuously applied to a liquid crystal layer so that the bend alignment is maintained. For example, as illustrated in FIGS. 16 and 17, in a case where (i) a white display is carried out when a voltage VL is applied, (ii) a black display is carried out when a voltage VH is applied, and (iii) an intermediate state is displayed when a voltage between VL and VH is applied, an alignment of the liquid crystal layer 12 is the bend alignment in a range of voltages VL through VH.
In the OCB mode, the liquid crystal layer 12 in the display state maintains the bend alignment while a voltage is consistently applied, whereas the alignment of the liquid crystal layer 12 is the splay alignment while a power of the liquid crystal display device is in an OFF state and no voltage is applied. On this account, when the power of the liquid crystal display device is turned on, an alignment transition from the splay alignment to the bend alignment (splay-to-bend transition) occurs in the liquid crystal layer 12.
However, it is known that the splay-to-bend transition requires a high voltage or a long time. It depends on a voltage applied to a liquid crystal layer how long it takes that the splay-to-bend transition is carried out over a screen. FIG. 18 illustrates how a voltage applied to a liquid crystal layer at room temperature (+25° C.) affects a transition time required for a splay-to-bend transition. In this case, an area of an electrode is 1 square centimeter (scm), and a cell thickness is 5 μm. As illustrated in FIG. 18, it is shown that, as the voltage applied to a liquid crystal layer increases, the splay-to-bend transition takes a shorter time.
Meanwhile, from observation of the splay-to-bend transition, it is shown that the transition occurs from a peculiar place where several spacers are nucleated. Such a place is called a transition nucleus. Since merely several transition nuclei may be generated in 1 scm, it takes longer that the splay-to-bend transition spreads over the entire screen. A spreading speed of the splay-to-bend transition depends on viscosity of a liquid crystal. For example, the viscosity of a liquid crystal largely increases at low temperature such as −30° C. In this case, the spreading speed of the splay-to-bend transition becomes ten times slower that that at room temperature.
Furthermore, in a practical TFT liquid crystal display panel, a pixel electrode is provided in a region surrounded by a source bus line and a gate bus line that are intersected with each other (hereinafter, both of a source bus line and a gate bus line are referred to as just bus lines). Generally, a distanced space is provided between the pixel electrode and the bus lines for insulating the pixel electrode from the bus lines.
In the distanced space, a voltage is not sufficiently applied to the liquid crystal layer because there are no pixel electrode and bus lines. This is shown in FIG. 19. FIG. 19 illustrates an electric potential of a liquid crystal layer when a voltage is applied to pixel electrodes, bus lines, and a counter electrode in a TFT liquid crystal display panel in which the pixel electrodes and the bus lines are provided in plane. As apparent from FIG. 19, in distanced spaces between the pixel electrodes and the bus line, a voltage is not applied to the liquid crystal layer.
As such, in a distanced space where no voltage is applied to a liquid crystal layer, even if a splay-to-bend transition occurs in a transition nucleus in a certain pixel electrode, the splay-to-bend transition does not spread into adjoining pixel electrodes over the distanced spaces. From this reason, the splay-to-bend transition thus occurred in a certain pixel electrode does not spread into pixel electrodes having no transition nucleus therein, which causes a problem in which the splay-to-bend transition does not spread over the entire screen.
In order to solve the problem, Patent Document 1 discloses an arrangement in which a convex section or a concave section made of a conductive material is provided at a specified position in a screen. With the arrangement, an electrical field intensity applied to a liquid crystal layer on the convex or concave section becomes larger than a surrounding area, thereby promoting generation of transition nuclei. Thus, the transition nuclei are formed in each pixel, with the result that the splay-to-bend transition is easily carried out in all the pixels.
Patent Document 2 discloses driving means for generating a potential difference between a first electrode (for example, an auxiliary capacitor electrode) and a second electrode (for example, a pixel electrode) that is provided so as to overlap the first electrode via an insulator and has a lacking section. With the arrangement, an electrical field intensity applied to between the two electrodes becomes larger than other regions, thereby resulting in that liquid crystal molecules positioned around the lacking section become transition nuclei. This facilitates a splay-to-bend transition to be carried out in all pixels.
In this way, in Patent Documents 1 and 2, all structures that are to be transition nuclei are provided in all pixels, so that, even if there are distanced spaces in which no voltage is applied to a liquid crystal layer, a splay-to-bend transition occurs in all pixels, i.e., over an entire screen.
[Patent Document 1]
Japanese Unexamined Patent Publication, Tokukaihei, No. 10-20284 (published on Jan. 23, 1998)
[Patent Document 2]
Japanese Unexamined Patent Publication, Tokukai, No. 2003-107506 (published on Apr. 9, 2003)